Approximately 65 million individuals worldwide are living with epilepsy. The first and most common form of relief relies on anti-epileptic drugs. However, one-fourth to one-third of the cases do not become seizure-free from drug therapy alone. In these situations, surgery may be an option if a single, localizable focus can be identified and safely removed.
To obtain a broad sense of the origins and types of seizures, neural activity is first monitored using an electroencephalography (EEG) system through scalp recordings of brain activity (Phase I). If the seizures appear to be potentially focal and unilateral, surgically implanted electrocorticographic (ECoG) electrodes on the cortical surface, or depth electrodes for deep foci, are used to monitor cortical activity during seizures and further define the epileptic foci (Phase II). The decision for surgical resection or intervention depends on the data from this invasive monitoring strategy that delineates the epileptogenic zone to ensure that resection of the seizure foci will not significantly impact neurological functions. During Phase II monitoring, in addition to seizure localization, several procedures are used to define areas of eloquent cortex, which is used to describe areas of cortex that—if removed—will result in loss of sensory processing or linguistic ability, minor paralysis, or paralysis, and attempt to estimate the cognitive functions possibly affected by respective surgery.
Electrical Cortical Stimulation (ECS) is considered the gold standard for sensorimotor functional delineation of eloquent tissue in the brain. In contrast to continuous monitoring where the electrical current from the brain is passively recorded, electrical current is passed between neighboring electrodes to evoke sensory or motor manifestations. Typically during ECS, 50 Hz square pulse trains are applied lasting two to five seconds. The stimulation current is gradually increased up to 10 mA until a sensory, a motor, or an after-discharge response is elicited.
A bottom-up approach can also be used for the mapping where peripheral nerves are electrically stimulated and evoked responses in the cortical signals are visually observed. However those two techniques have limitations. The somatic response is subjective and interpretative based on the subject's response and direct observation by the tester. Cortical stimulation does not always elicit motor responses in children under ten years of age. For sensory areas, it is often difficult to interpret evoked stimuli. In children, particularly those who are too young or non-verbal due to cognitive dysfunction, interpretation of sensation can be very difficult. Additionally, after-discharges, an unwanted consequence of ECS stimulation, are frequent, and can lead to seizures.
Therefore, it would be desirable to have a system and method for cortical mapping that does not use electrical stimulation but produces resolution comparable with that with electrical stimulation, or require detailed or any communication with the patients during the mapping.